Wiring, Harnessing and Circuit Protection
Wiring, Harnessing and Circuit Protection
This domain covers cable selection, voltage drop, fuses, fusible links, circuit breakers, relays, terminals, harness routing and connector protection.
Core rules
- Cable must carry the load current with acceptable voltage drop, usually under about 0.5 V for a lighting or accessory circuit and less for sensors.
- Fuse rating is chosen to protect the cable and is normally above the steady load current but below the cable capacity.
- Fuses are fast; fusible links protect battery feeds; circuit breakers may be manual or automatic.
- Relays let a low-current switch control a high-current load. Typical ISO relay terminals: 30 battery feed, 87 load, 85 and 86 coil, 87a normally closed.
- Good crimping, correct terminal size, heat-shrink, grommets and sealed connectors prevent corrosion and fatigue.
- Harness routing must avoid heat, sharp edges, moving parts and water traps; provide drip loops and strain relief.
Worked example
An 8 A accessory is 5 m from the battery, so the round-trip cable length is 10 m. Allow 0.5 V drop. R = V/I = 0.5/8 = 0.0625 Ω. Using copper resistivity 0.0175 Ω·mm²/m, A = ρL/R = (0.0175 × 10)/0.0625 = 2.8 mm². Select the next standard cable size, 3 or 4 mm², and a fuse above 8 A but within the cable rating, such as 10 A, after checking the cable free-air rating.
Traps
Fitting a larger fuse to stop it blowing; using a fuse rated for the device rather than the cable; twisting wires instead of crimping; leaving a relay without a suppression diode across its coil; routing harnesses against exhaust or suspension; ignoring voltage drop on long runs; using the body as a return without proper bonding.